safety in a 15 passenger van

We have 8 children and drive a Dodge 15 passenger van. Our van is only equipped with 4 shoulder belts for the bench seats. There are no shoulder belts down the side behind the front passenger seat. We have four above the age 8 who do not require or fit in carseats. We will soon have others who will not fit in a carseat. How do they ride safely without a shoulder strap? We have the lap belts for all the seats but these are not enough. Any suggestions?

I would put the 4 kids big & mature enough for lap/shoulder belts along that side with those belts -- many highback boosters go up to 100 pounds & if you answer "yes" to ALL these questions then they don't need a boooster:
The 5-Step Test
1. Does the child sit all the way back against the auto seat?
2. Do the child's knees bend comfortably at the edge of the auto seat?
3. Does the belt cross the shoulder between the neck and arm?
4. Is the lap belt as low as possible, touching the thighs?
5. Can the child stay seated like this for the whole trip?
[Britax Bodyguard is a great booster offering side-impact protection*]

As for the lap-only belts with your younger children, I would think you should probably buy carseats that will hold bigger kids -- like the Britax Husky > http://www.car-seat.org/showthread.php?s=&threadid=553
I've heard that some areas offer vouchers for carseats ... you can ask your local CPS tech about that & for more help on properly securing your children in your vehicle > http://www.nhtsa.dot.gov/CPS/CPSFitting/Index.cfm

Also, there is the E-Z-ON Universal Harness for your kids not in carseats & without shoulder belts! > http://www.ezonpro.com/products/familyVehicle/86yHarness.shtml

*I'm sure that with 8 kids a 15-passenger van is about the only option, but here's this if you haven't already seen > http://www.nhtsa.gov/cars/problems/studies/15PassVans/15PassCustomerAdvisory.htm

Hope this helps & good luck

Just clarifying that I meant the kids who can use shoulder belts should ... I think I misread the OP -- thought before that the side behind the driver had shoulder belts, but where ever they are I believe the kids old enough & mature enough should use them & the younger kids would need E-Z-Ons or a seat like the Husky.